The invention relates generally to signal correction and more particularly to correcting DC offset in the output of a frequency to voltage converter such as that used in the protection circuit of a motor drive.
The use of motor drives for controlling the speed and/or torque of motors, such as AC induction motors, is well known. Typically, an AC motor drive includes a rectifier section for converting an AC input voltage into a DC voltage for charging a DC bus and an inverter section comprising switching bridges for generating a three phase AC output signal, or an approximation thereof that cause sinusoidal current in the motor, for driving a three phase AC motor. The switching bridges include switching elements that can be controlled in a known manner to vary the frequency of the output signal to thereby control the motor in a desired manner.
Known motor drives include various protection functions and other control functions. Often these functions are accomplished by a microprocessor based controller programmed in a desired manner or analog circuits. For example, it is known to provide regulation of the DC bus voltage for the purpose of overvoltage protection during regenerative braking and/or operation anomalies. It is desirable to electrically isolate the regulation circuit from the DC bus to prevent damage to the regulation circuit. Accordingly, it is known to use a voltage to frequency converter to convert the DC bus voltage to a signal having a frequency that is related to the DC bus voltage, to convert the frequency signal to light pulses, and to transmit the light pulses over a fiber optic link to the protection circuit. A frequency to voltage converter associated with the protection circuit converts the frequency signal back to a voltage signal to permit voltage regulation in a known manner.
There are two conventional types of protection circuits. The first type of protection circuit is an analog circuit that uses known analog components to convert the frequency signal to a voltage signal. Analog circuits are very responsive, i.e. fast. However, analog circuits have an inherent DC offset. In fact, it is difficult to reduce the analog offset to less than .+-.2%, even when using high grade, expensive components. In many applications, the error due to the DC offset is unacceptable.
The second type of protection circuit is a digital circuit that uses a programmed microprocessor based device and interface components to convert the frequency signal to a voltage signal. Digital protection circuits can be very accurate and are not subject to DC offset problems associated with analog circuits. However, a digital protection circuit must accumulate or sample the pulses of the frequency signal and thus is inherently unresponsive, i.e. slow.